Mine are Half Life and Morrowind.

I feel like back when I didn't have a massive game library I would have spent so much time replaying Half Life and exploring every inch of the maps and trying to find exploits to break the game. The massive modding community likely would have kept me playing for years.

Morrowind probably would have gotten me a lot more into RPGs and fantasy games. I do love RPGs but still struggle to immerse myself in fantasy settings. I assume this is because I mostly played SciFi games growing up.

What about you? Is there an MMO or something you maybe tried and gave up on to soon?